Which of the following is NOT a part of a flower?

The correct answer is C, +7. Potassium permanganate is a neutral compound, so the oxidation numbers of all its atoms must add up to zero. Potassium, an alkali metal, is always +1, and each of the four oxygen atoms is -2, giving the oxygens a total of -8. Writing the balance as 1 plus x minus 8 equals 0 gives x as +7, so manganese is in the +7 state. This is the highest oxidation state manganese can reach, because it has seven electrons available in its 3d and 4s shells, and it is why KMnO4 is such a strong oxidising agent, used to disinfect water and to titrate in the laboratory. Option A is wrong because +2 is the state of manganese in MnCl2 or MnSO4. Option B is wrong because manganese is a metal and does not take a negative state in this salt. Option D is wrong because a -7 state is impossible for a transition metal here. Exam tip: in KMnO4 manganese is +7, and in K2Cr2O7 chromium is +6.

The correct answer is A, Two dry surfaces. Coulomb friction, also called dry friction, is the resistance that acts when two solid surfaces touch each other with no lubricant between them. Charles-Augustin de Coulomb set out its rules: the friction force is proportional to the normal reaction, it does not depend on the apparent area of contact, and it is very nearly independent of the sliding speed. Dry friction covers both the static friction that must be beaten before motion can start and the kinetic friction that acts once sliding has begun, and the limiting static value is the larger of the two. Option B is wrong because friction between lubricated surfaces is called fluid or film friction and obeys the laws of viscosity instead. Option C is wrong because relative motion describes kinetic friction, which is only one part of Coulomb friction. Option D is wrong because the resistance between a solid and a liquid is viscous drag. Exam tip: Coulomb friction means dry friction, proportional to the normal reaction and independent of the contact area.

The correct answer is C, Cleaning. In electroplating the article to be coated is made the cathode, the plating metal serves as the anode, and both are dipped in an electrolyte containing a salt of that metal. When current flows, the metal ions travel to the cathode and settle on it. The coating can bond only if the surface is chemically clean, so grease, oil, rust and oxide are stripped off first by degreasing and by pickling in acid; any film left behind makes the plating peel, blister or come out patchy. Option A is wrong because buffing only brings up a shine on a surface that is already clean. Option B is wrong because soldering joins two metals with a filler alloy and has nothing to do with preparing a surface for plating. Option D is wrong because polishing smooths the surface mechanically but does not remove the oily and oxide films that spoil adhesion. Exam tip: the order is clean first, then plate, and the article is always the cathode.

The correct answer is C, Ear. Balance is controlled by the inner ear, which holds the vestibular apparatus alongside the organ of hearing. It is made of three semicircular canals set at right angles to one another and two small sacs, the utricle and the saccule. The canals are filled with fluid; when the head turns, the fluid lags behind and bends tiny hair cells, which send the brain a signal about the direction and the speed of the movement. The utricle and the saccule report the pull of gravity and straight-line motion. This is why an inner-ear infection, or spinning round quickly, leaves a person giddy. Option A is wrong because the heart only pumps blood through the body. Option B is wrong because the liver makes bile, breaks down toxins and stores glycogen. Option D is wrong because the tongue carries the taste buds and helps in speech and swallowing. Exam tip: the cochlea of the inner ear is for hearing, and the three semicircular canals are for balance.

The correct answer is A, 400 m/s. The speed of any wave is its frequency multiplied by its wavelength, so here 200 Hz times 2 m gives 400 metres per second. Frequency is the number of complete waves that pass a point each second, measured in hertz, and wavelength is the distance covered by one complete wave; multiplying the two therefore gives distance per second, which is speed. In a given medium the speed of sound is fixed by the medium itself, so when the frequency rises the wavelength must fall in the same proportion. Sound travels at roughly 343 m/s in air at 20 degrees Celsius, about 1,500 m/s in water and about 5,000 m/s in steel, because particles in a denser and more elastic medium pass the vibration along faster. Option B is wrong because 200 is the frequency copied without multiplying. Option C is wrong because 80 does not follow from these numbers at all. Option D is wrong because it divides the frequency by 2 instead of multiplying. Exam tip: speed equals frequency times wavelength, and in air sound moves at about 343 m/s.